1. Field of the Invention
The present invention relates to an engine throttle valve control device for a vehicle.
2. Description of the Related Art
In a conventional engine throttle valve control device for a vehicle a throttle opening angle controlled by an electronic control system is set to one which is optimal running speed of a vehicle in the cruise control mode or driving the ordinary running of a vehicle, as is exemplified by the device shown in FIGS. 17 and 18.
In those Figures, a throttle valve 101 is secured to a throttle shaft 102 which is rotatably supported by a throttle body 103. An end of the throttle shaft 102 adjacent to an accelerator pedal 108 is engageable with an accelerator upper limit lever 106. The throttle shaft 102 is biased by springs 104 and 105 in a direction which opens the throttle valve 101 (hereinafter referred to as the valve opening direction). The accelerator upper limit lever 106 is biased by a spring 107 in the direction which closes the throttle valve 101 (hereinafter referred to as the valve closing direction). The other end of the throttle shaft 102 which is adjacent to a motor 120 is arranged to abut a motor-side guard 121 when it moves in the valve opening direction. The motor-side guard 121 is biased by a spring 122 in the valve opening direction.
During the normal operation of a vehicle, the accelerator upper limit lever 106 opens or closes according to the degree of pedal operation, and the throttle valve 101 opens or closes according to the operation of the accelerator upper limit lever 106. The valve position corresponding to the throttle valve open angle (hereinafter referred to as the open angle) of the motor-side guard 121 is controlled based on signals from a throttle valve position sensor 131 and an accelerator position sensor 132. The motor-side guard 121 causes the throttle valve 101 when moving in the valve closing direction.
During the cruise control, a diaphragm in a diaphragm actuator 110 is pulled by a vacuum pressure to move the accelerator upper limit lever 106 to the maximum valve opening position as shown in FIG. 18. The motor-side guard 121 is driven by the motor 120 and causes the throttle valve 101 in the valve closing direction in the same manner as in normal operation.
In the conventional throttle valve control device as described above, however, when the cruise control is initiated, the accelerator upper limit lever 106 is driven by the diaphragm actuator 110 in the valve opening direction against the biasing force of the spring 107, and therefore a large-sized diaphragm actuator 110 is required to generate a sufficiently strong driving force. Further, since the accelerator upper limit lever 106 is at its full extent of movement, when the driver operates the accelerator pedal 108 to further increase the vehicle speed, the pedal operation must be detected by the valve sensor 131 and the accelerator sensor 132, thereby resulting in complicated processing and increased response time.
There is another conventional throttle valve control device disclosed in U.S. Pat. No. 5,092,296. In that device, a motor driving force is transmitted to the throttle valve through an electromagnetic clutch which is disposed coaxially with the throttle shaft. The device has a reduction gear mechanism to reduce the rotational speed at a certain gear ratio and, therefore, an increased rotational torque is transmitted to the throttle shaft. As a result, an increased electromagnetic force is required for transmitting the driving force, thereby increasing the size of the electromagnetic clutch.
Japanese Patent Application Laid Open No. Hei 6-833804 discloses another conventional throttle valve control device which has a relatively small electromagnetic clutch disposed coaxially with a motor shaft. In this device, it is necessary to provide a member to carry the clutch member rotatably on the motor shaft. This structure still increases the size of the electromagnetic clutch.